This disclosure relates to fracturing operations in subterranean formations, and more specifically, to low-polymer-load fracturing fluids that comprise degradable fibers treated with crosslinkable, hydratable polymers that form a gelled network in the fluid, and methods relating thereto.
Fracturing fluids are used in hydraulic fracturing operations in subterranean formations to stimulate the production of oil, gas and other fluids from the formation by means of increasing the permeability or conductivity thereof. Fracturing fluids have to be carefully designed to meet the rheological specifications necessary to ensure the desired performance of the fluid. In some instances, the fluids should have sufficient viscosity to create and propagate fractures in the formation matrix as well as carry and place proppant particulates into the fractures. Oftentimes this is achieved by including a polymeric gelling agent (e.g., biopolymers such as guar and xanthan or synthetic polymers such a polyacrylamides) in the fracturing fluid, which gels the fracturing fluid downhole to a gelled viscosity through formation of a viscous polymeric network in the fluid. A large concentration of polymer is often required to achieve the requisite viscosity, for example, 25 lbs./1000 gal up to 60 lbs./1000 gal of the fracturing fluid. These fluids are considered “high-polymer-load fracturing fluids,” as that term is used herein. Particularly for high-temperature wells, it is often necessary to crosslink the polymer gelling agent (e.g., with a suitable crosslinking agent) to ensure that the gelled viscosity has sufficient strength and for fracturing applications. The resulting crosslinked polymer has an even higher gelled viscosity, which is thought to be even more effective at carrying proppant into the fractured formation. Borate crosslinking agents have been used extensively, typically in high pH fluids, for guar, guar derivatives and other galactomannans. Other crosslinking agents include, for example, titanium, chromium, iron, aluminum, and zirconium crosslinking agents.
In some instances, after the proppant particles are placed in the fractures, the viscosity of the gelled fluid used to carry the proppant particles is reduced by breaking the gelled fluid. Unfortunately, when the gelled fluid is broken downhole, these polymeric gelling agents leave gel residue in the propped fractures and/or in the formation that can reduce the formation's permeability and/or the conductivity of the propped fractures. Gel residue is often a product of an inefficient break, but also may be due to the inherent nature of the polymers, which is exacerbated by the high concentration used in common high-polymer-load fracturing fluids. Additionally, an increasing amount of insoluble residue may result if the gel has been crosslinked. Even in those breaks that are thought to be efficient (e.g., fully broken), insoluble residues that are part of the gel system may be left in the formation that can also foul the formation. Guar is an example of a commonly used polymeric gelling agent that is known to leave up to 10% insoluble residue in the formation causing formation damage (e.g., reduced formation permeability and fracture conductivity). These residues are very difficult to remove, and often are left in the formation, consequently adding to the well's negative environmental footprint.
Once the proppant particulates have been placed in the fractures, an ideal fracturing fluid should revert to a low viscosity fluid (i.e., a fluid having a lower viscosity than its gelled viscosity) that can easily be removed from the propped fracture(s) to facilitate high production of hydrocarbons without leaving insoluble products or residues that may impede the formation permeability or conductivity of the propped fractures to significantly impact the production of the well.